In recent years, power generation by natural energy such as sunlight or wind power has found use.
FIG. 15 shows an example in which an existing power system (a system superior to a distribution system) 1 and a distribution system (micro grid) 10 are connected via a line impedance Ls and a circuit breaker 2.
A dispersed generation plant 11 and a load 12 are connected to the distribution system 10 which is the micro grid. The dispersed generation plant 11 is illustrated as a single generator in FIG. 15. Actually, however, it is composed of a plurality of dispersed facilities for power generation, which include natural energy type power generation equipment utilizing natural energy (e.g., photovoltaic power generation equipment or wind power generation equipment), and internal combustion engine type power generation equipment driven by an internal combustion engine (e.g., diesel power generation equipment). Also, the load 12 is actually a plurality of dispersed loads.
With the micro grid 10 as shown in FIG. 15, the amount of power generation varies or fluctuates greatly according to weather conditions, wind speed, etc., because it has natural energy type power generation equipment.
In order to absorb or accommodate such fluctuations in the amount of power generation, therefore, a system stabilizing device is used.
With the internal combustion engine type power generation equipment, output power is adjusted by governor control. However, governor control is slow in response. Thus, if electric power consumed by the load 12 suddenly changes, the internal combustion engine type power generation equipment cannot follow such a sudden change (sudden excess or deficiency) in electric power.
The system stabilizing device is also used for the purpose of following such a sudden change in electric power with good response, thereby assisting the internal combustion engine type power generation equipment to balance demand for and supply of electric power.
The system stabilizing device is a power converter having a power storage function, and it is also a device installed in the distribution system to make the aforementioned power compensation.
FIG. 16 shows an example in which a system stabilizing device 20 is provided in the distribution system (micro grid) 10 shown in FIG. 15. The system stabilizing device 20 is connected in parallel with the dispersed generation plant 11 and the load 12.
The system stabilizing device 20 has a control unit 21, a power converter 22 (inverter) capable of an inverting action and a converting or rectifying action, and a direct current charging unit 23, such as an electric double layer capacitor or a storage battery (battery), as main members.
The power converter 22 acts responsive to agate signal g fed from the control unit 21. This power converter 22, when performing a converting action, converts an alternating current (AC) power obtained from the distribution system 10 into a direct current (DC) power, and charges this DC power into the direct current charging unit 23. When performing an inverting action, the power converter 22 converts the DC power charged in the direct current charging unit 23 into an AC power, and sends this AC power to the distribution system 10.
The power outputted from the power converter (inverter) 22 is passed through a filter circuit 27, and sent out to the distribution system 10. That is, the power sent from the system stabilizing device 20 out to the distribution system 10 is the power outputted from the power converter 22 and then filtered by the filter circuit 27.
The filter circuit 27 is composed of a reactor, a capacitor and a transformer, and functions to smooth a pulse voltage outputted from the power converter 22.
In the system stabilizing device 20, a system current Is, which flows from the power system 1 into the distribution system 10, is detected by a current detector 24, a system voltage Vs which is the voltage of the distribution system 10 is detected by a voltage detector 25, and a converter current IINV inputted to and outputted from the power converter 22 is detected by a current detector 26. Moreover, a current detector AA for detecting a current fed out of the system stabilizing device 20 to the distribution system 10 is provided for reasons to be described later.
Under normal conditions where no breakdown or the like occurs in the power system 1, the circuit breaker 2 is in a connected state, so that “a system-interconnected run”, an operation performed with the distribution system 10 being tied to the power system 1, is performed in the system stabilizing device 20. During the system-interconnected run, electric power is supplied to the load 12 by the power system 1, the dispersed generation plant 11, and the system stabilizing device 20.
Under abnormal conditions where a breakdown occurs in the power system 1, on the other hand, the circuit breaker 2 is in a cut-off state, and the system stabilizing device 20 makes a “self-supporting or isolated run”, an operation performed with the distribution system 10 being cut off from the power system 1. During the self-supporting run, electric power is supplied to the load 12 by the dispersed generation plant 11 and the system stabilizing device 20.
The system stabilizing device 20 performs the following actions during the system-interconnected run and the self-supporting run:
(1) During the system-interconnected run, the system stabilizing device 20 acts to detect the system current Is flowing into the distribution system 10, determine a system power from the system current Is, and suppress fluctuations in this system power.
(2) During the self-supporting run, the system stabilizing device 20 detects the system voltage Vs within the distribution system 10, and performs a compensating action so that the voltage amplitude and frequency of this system voltage Vs become stable.
Details of the control unit 21 of the system stabilizing device 20 will be described by reference to FIG. 17.
A phase-locked loop (PLL) 101 outputs a reference phase signal θ showing the phase of the system voltage Vs based on the system voltage Vs. A sine wave generator 102 outputs a three-phase voltage waveform {sin(θ), sin(θ−2/3π), sin(θ+2/3π)}, corresponding to a rated voltage synchronized to the reference phase signal θ, as a reference three-phase sine wave signal K.
A change-over switch 103, during the system-interconnected run, has movable contacts 103a, 103b thrown to the A side as indicated by solid lines in the drawing and, during the self-supporting run, has the movable contacts 103a, 103b thrown to the B side as indicated by dashed lines in the drawing.
Next, explanations will be offered for the respective functional blocks working during the system-interconnected run, and for their control actions during the system-interconnected run.
A dq transformation unit 104 dq-transforms the system current Is to a rotating coordinate system rotating in a phase indicated by the reference phase signal θ to output the active component Isd of the system current and the reactive component Isq of the system current.
A first fluctuation detecting block 105 detects the fluctuation component of the active component Isd of the system current on the dq-axes, and outputs it as a current command Irefd for the active component. A second fluctuation detecting block 106 detects the fluctuation component of the reactive component Isq of the system current on the dq-axes, and outputs it as a current command Irefq for the reactive component.
The fluctuation detecting blocks 105, 106 are band-pass filters having a differentiation function and a filter function, and details of their structures will be described later.
A dq transformation unit 107 dq-transforms the converter current IINV to a rotating coordinate system rotating in a phase indicated by the reference phase signal θ to output the active component Iinvd of the converter current and the reactive component Iinvq of the converter current.
A subtraction unit 108 subtracts the active component Iinvd of the converter current from the current command Irefd for the active component to output a current deviation Δd for the active component. A subtraction unit 109 subtracts the reactive component Iinvq of the converter current from the current command Irefq for the reactive component to output a current deviation Δq for the reactive component.
A current control unit 110 exercises the proportional plus integral (PI) control of the current deviation Δd for the active component to output a voltage command Vd for the active component. A current control unit 111 exercises the proportional plus integral (PI) control of the current deviation Δq for the reactive component to output a voltage command Vq for the reactive component.
A dq inverse transformer unit 112 applies dq inverse transformation to the voltage command Vd for the active component and the voltage command Vq for the reactive component to output a voltage command Vφ of a fixed coordinate system.
An addition unit 113 adds the reference three-phase sine wave signal K to the voltage command Vφ to output a final voltage command V*.
A PWM (pulse width modulation) modulator 114 PWM-modulates the voltage command V* to output the gate signal g.
In accordance with this gate signal g, action control over the power converter 22 is effected. To suppress fluctuations in the system current Is during the system-interconnected run, power is outputted from the power converter 22.
Next, explanations will be offered for the respective functional blocks working during the self-supporting run, and for their control actions during the self-supporting run.
A frequency detection unit 121 detects the frequency of the system voltage Vs to output a frequency signal F. The frequency of the system voltage Vs corresponds to the active power, and is in a corresponding relationship with the active power such that when the active power decreases, the frequency of the system voltage Vs decreases, and when the active power increases, the frequency of the system voltage Vs increases.
An amplitude detection unit 122 detects the amplitude of the system voltage Vs to output an amplitude signal L. The amplitude of the system voltage Vs corresponds to the reactive power, and is in a corresponding relationship with the reactive power such that when the reactive power decreases, the amplitude of the system voltage Vs decreases, and when the reactive power increases, the amplitude of the system voltage Vs increases.
A third fluctuation detecting block 123 detects the fluctuation component of the frequency signal F, and outputs it as a current command Irefd for the active component. A fourth fluctuation detecting block 124 detects the fluctuation component of the amplitude signal L, and outputs it as a current command Irefq for the reactive component.
The fluctuation detecting blocks 123, 124 are band-pass filters having a differentiation function and a filter function, and details of their structures will be described later.
The subtraction unit 108 subtracts the active component Iinvd of the converter current from the current command Irefd for the active component to output a current deviation Δd for the active component. The subtraction unit 109 subtracts the reactive component Iinvq of the converter current from the current command Irefq for the reactive component to output a current deviation Δq for the reactive component.
The current control unit 110 exercises the proportional plus integral (PI) control of the current deviation Δd for the active component to output a voltage command Vd for the active component. The current control unit 111 exercises the proportional plus integral (PI) control of the current deviation Δq for the reactive component to output a voltage command Vq for the reactive component.
The dq inverse transformer unit 112 applies dq inverse transformation to the voltage command Vd for the active component and the voltage command Vq for the reactive component to output a voltage command Vφ of a fixed coordinate system.
The addition unit 113 adds the reference three-phase sine wave signal K to the voltage command Vφ to output a final voltage command V*.
The PWM (pulse width modulation) modulator 114 PWM-modulates the voltage command V* to output the gate signal g.
In accordance with this gate signal g, action control over the power converter 22 is effected. To suppress fluctuations in the voltage amplitude and frequency of the system voltage Vs during the self-supporting run, power is outputted from the power converter 22.
The fluctuation detecting blocks 105, 106, 123, 124 are composed of the band-pass filters, as stated above.
The configuration of a conventional band-pass filter 50, which can be used as the fluctuation detecting blocks 105, 106, 123, 124, will be described by reference to FIG. 18. In FIG. 18, the symbol s denotes a Laplace operator showing a differentiation function.
As shown in FIG. 18, this band-pass filter (fluctuation detecting block) 50 is composed of a low-pass filter 51, a low-pass filter 52, and a subtracter 53.
The pass band frequency of the band-pass filter 50 is determined in accordance with filtering characteristics required of the respective fluctuation detecting blocks 105, 106, 123, 124. The cut-off frequency on the high frequency side of the determined pass band frequency is set to be f1, and the cut-off frequency on the low frequency side of the determined pass band frequency is set to be f2. The time constant of the low-pass filter 51 for noise removal, which has a cut-off frequency set at f1, is T1. The time constant of the low-pass filter 52 for setting the fluctuation detection time, which has a cut-off frequency set at f2, is T2.
The low-pass filter 51 is a filter having first order lag characteristics whose time constant is set to be the time constant T1 determined for the purpose of noise removal.
The low-pass filter 52 is a filter having first order lag characteristics whose time constant is set to be the time constant T2 determined for the purpose of setting the time for detecting fluctuations.
Upon entry of an input signal, both filters 51 and 52 utilize their filtering characteristics to filter the input signal.
The input signal is the active component Isd of the system current, if the band-pass filter (fluctuation detecting block) 50 is the fluctuation detecting block 105. The input signal is the reactive component Isq of the system current, if the band-pass filter (fluctuation detecting block) 50 is the fluctuation detecting block 106. The input signal is the frequency signal F, if the band-pass filter (fluctuation detecting block) 50 is the fluctuation detecting block 123. The input signal is the amplitude signal L, if the band-pass filter (fluctuation detecting block) 50 is the fluctuation detecting block 124.
The subtracter 53 outputs a signal obtained by subtracting the output signal of the low-pass filter 52 from the output signal of the low-pass filter 51. The signal outputted from the subtracter 53 is a fluctuation component signal.
This fluctuation component signal is the current command Irefd for the active component which is the fluctuation component of the active component Isd of the system current, if the band-pass filter (fluctuation detecting block) 50 is the fluctuation detecting block 105. The fluctuation component signal is the current command Irefq for the reactive component which is the fluctuation component of the reactive component Isq of the system current, if the band-pass filter (fluctuation detecting block) 50 is the fluctuation detecting block 106. The fluctuation component signal is the current command Irefd for the active component which is the fluctuation component of the frequency signal F, if the band-pass filter (fluctuation detecting block) 50 is the fluctuation detecting block 123. The fluctuation component signal is the current command Irefq for the reactive component which is the fluctuation component of the amplitude signal L, if the band-pass filter (fluctuation detecting block) 50 is the fluctuation detecting block 124.